Add an index on derivation_output_details.id

Where hash is not null, since this helps with the fixed output package
derivations query.

sqitch/verify/package_derivations_not_null_hash_index.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:package_derivations_not_null_hash_index on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;